Equalization permits communication devices communicating with one another via communication signals on a communication channel to compensate for linear distortion impairments experienced with the communication signals on the communication channel. Examples of linear distortion impairments include micro-reflections (a short time delay echo or reflection caused by an impedance mismatch), group delay variation (a difference in propagation time where some frequency components of a signal may arrive before others), in band amplitude ripple/tilt (a non-flat frequency response), and the like.
Adaptive equalizers can be utilized to compensate for linear distortion impairments. The adaptive equalizers accomplish equalization by developing a digital filter that reverses or negates the affects of the linear distortion and that has approximately the opposite complex frequency response of the channel through which a desired signal is transmitted. An example of a communication system that may use equalization techniques and adaptive equalizers is a conventional cable modem system.
A typical cable modem system includes a network providing point-to-multipoint topology for supporting data communication between a Cable Modem Termination System (CMTS) at the cable headend and multiple cable modems at the customer premises. The headend is a central facility that is used for receiving, processing, and combining broadcast, narrowcast and other signals to be carried on the cable network. In such systems, information is broadcast downstream from the CMTS at the headend to the cable modems as a continuous transmitted signal in accordance with a time division multiplexing (TDM) technique. In contrast, information is transmitted upstream from each of the cable modems to the CMTS as short burst signals in accordance with a time division multiple access (TDMA) technique.
The upstream transmission of data from the cable modems is managed by the CMTS, which allots to each cable modem specific slots of time within which to transfer data. Conventional cable modem systems utilize DOCSIS-compliant equipment and protocols to carry out the transfer of data packets between multiple cable modems and the CMTS. The term DOCSIS (Data Over Cable System Interface Specification) refers to a group of specifications that define industry standards for cable headend and cable modem equipment.
Within the DOCSIS standard, the upstream transmission (from the subscriber premises toward the network and CMTS) is a burst type of communication and the linear distortion affecting each communication is unique to each subscriber site/modem. As a result, the CMTS of such a system must adapt its adaptive equalizer to match the unique characteristics of each subscriber's transmission. This process is referred to as “post-equalization” and requires longer preamble transmissions for burst type upstream transmissions prior to the actual data passing for the post-equalization to be effective. The longer preambles result in increased overhead.
The DOCSIS specification permits the increased overhead demanded by post-equalization to be countered and reduced by providing for a “pre-equalization” process. Pre-equalization is a process by which the CMTS communicates a unique digital filter to an adaptive pre-equalizer of each subscriber modem. The adaptive pre-equalizer is a circuit or the like in the subscriber modem that pre-equalizes or pre-distorts upstream signals transmitted by the modem to compensate for expected and pre-measured linear impairments. Accordingly, the adaptive pre-equalizer of the modem utilizes the digital filter information provided by the CMTS to enhance any successive upstream transmissions from the modem to the CMTS. The subscriber modem is then able to send its data bursts without the overhead of the long preamble.
The pre-equalization process requires the CMTS at the headend to measure and estimate the distortion of communication signals received from each subscriber modem and to then determine unique filter equalization coefficients for each modem needed to compensate for the measured distortion. Thereafter, the unique filter equalization coefficient or a subsequent update thereof is relayed to each modem to update equalization coefficient settings of the adaptive pre-equalizer of the particular modem. The digital filter created is then applied to all future burst communication signals transmitted by the modem to the CMTS until the modem receives an update or a further update from the CMTS. In this manner, linear distortion of communication signals is properly compensated and the signals received by the CMTS require less or no post-equalization.
A problem can arise with the above referenced pre-equalization process when equalization coefficients or updates thereof are improperly determined by the CMTS due to impulse noise or like transient impairment present on the communication channel when the CMTS measures and determines distortion experienced on the communications channel. During the CMTS measurement process, a temporary impairment such as impulse noise or the like can significantly corrupt the digital filter that is derived by the CMTS and that is subsequently sent to the subscriber modem. If this occurs, any subsequent data transmissions from the modem will have errors due to the use of improper equalization coefficients and possibly all communications may be lost between the CMTS and modem requiring re-registration of the modem.
Communication techniques have been derived with respect to minimizing the effects of impulse noise; however, these techniques are directed to correcting the data burst transmission through various coding schemes. For example, Reed-Solomon Forward Error Correction (RS FEC) is a method of error detection and correction in which redundant information is sent with a data payload in order to allow the receiver to reconstruct the original data should an error occur during transmission. This type of correction occurs at a higher layer in the communications stack above the physical layer. In contrast, equalization occurs at the physical layer (i.e., the actual signal communications step). When considering corruption by a temporary event such as impulse noise, algorithms such as RS FEC or coding gain (achieved with spread communication technologies such as SCDMA) can be used to correct the affects of the single unique event upon the single data transmission. However, in the case of technologies such as pre-equalization where measurements are taken to derive a compensation filter for subsequent future transmissions, often many subsequent data transmissions, the affect of that one temporary noise event may subsequently destroy many more data transmissions.